1. Field of the Invention
The present invention relates to an unvulcanized tread material for a pneumatic tire used in vehicles such as automobiles, a method of manufacturing a pneumatic tire by using the unvulcanized tread material for a pneumatic tire, and a pneumatic tire obtained by this method.
2. Description of the Related Art
Conventionally, a cap/baseplate structure has been proposed as a means to reconcile reciprocal properties of a pneumatic tire, such as wet resistance and low-rolling resistance.
A cap/baseplate structure for radial tires for, in particular, heavy loads has been proposed in which a combination of a cap rubber, formed of an NR/SBR rubber which may provide irregular wear resistance, and a base rubber, formed of an NR rubber which may provide heat build-up resistance and crack growth resistance, is used.
A conventional unvulcanized tread material having a cap/base structure is illustrated in FIG. 6. An unvulcanized tread material 100 is subject to extrusion molding by an extruder to be formed as an elongated sheet. As shown in the drawing, the unvulcanized tread material 100 is a two-layer structure in which a cap rubber 102 and a base rubber 104 are laminated. A boundary surface 106 between the cap rubber 102 and the base rubber 104 is substantially plane-shaped. Further, a cross section orthogonal to the longitudinal direction of the unvulcanized tread material 100 is a trapezoidal shape in which the tread side (the upper side of FIG. 6) has a narrow width.
FIG. 7 is a partially sectional view of a tire obtained by vulcanizing in a mold the green tire having the tread material 100. As shown in FIG. 7, in a manufactured tire 108, the boundary surface 106 of the cap rubber 102 and the base rubber 104 is curved in at vicinities of bottom portions of circumferential direction grooves 112 of a tread 110. Portions of the cap rubber :102 remain as thin-walls so as to surround bottom portions 112A of the respective circumferential direction grooves 112.
When the manufactured tire 108 is put into actual use, stress is often concentrated on the bottom portions 112A of the circumferential direction grooves 112, especially those on the shoulder portion, due to curbs and the like which the vehicle rides over. In the conventional manufactured tire 108, because the periphery of the bottom portion 112A is formed by the cap rubber 102, cracks form easily in the bottom portion 112A. There is therefore a drawback in that the cracks may spread to the boundary surface 106 and cause rib tear defects.